The present invention relates to a line driver and an amplifying method in said line driver.
Multi-carrier modulation is a known method for transmitting broadband information (for example, video, Internet or telephony) over radio connections or copper wire. The latter may be e.g. xDSL systems, such as Asymmetric Digital Subscriber Line (ADSL), High-rate Digital Subscriber Line (HDSL) or Very high speed asymmetric Digital Subscriber Line (VDSL). Two similar methods in multi-carrier modulation are Orthogonal Frequency Division Multiplex (OFDM), used in radio applications, and Discrete Multitone (DMT), which is used in copper wires.
Very briefly explained, the bits that are to be transmitted, (of for example a digitally encoded video signal) are encoded as complex numbers in a transmitter. In the transmitter an Inverse Fast Fourier Transform (IFFT) and a digital-to-analogue conversion are carried out whereupon the result is sent out on a line to a receiver.
The IFFT-modulation gives a sum of orthogonal carriers or tones, the amplitudes and phase displacement of which are determined by the values and phases of the complex numbers. These carriers are then transmitted in time slots at constant time intervals and are called symbols. In the receiver an analogue-to-digital conversion and a Fast Fourier Transform (FFT) are carried out instead. In this way, the original bits are retrieved. Attenuation and phase displacement may be easily compensated for, by multiplication by a complex number for each carrier.
In an xDSL system there is a line driver after the digital-to-analogue conversion in the transmitter. Said line driver is an amplifier that feeds the line. Since the output from the IFFT-modulation approximately is Gaussian distributed, the peak-to-average ratio is very high. This means that the line driver must have a high supply voltage in order to adequately transmit the occasional high signal peaks that may occur.
Unfortunately, such a high supply voltage results in substantial power dissipation in the line driver. In fact, e.g. in a typical commercial ADSL-system, about 67% of the total power is consumed in the line driver. Thus, there is a need to reduce the power dissipation in such a line driver. Power dissipated in digital logic will be possible to reduce in the future by improved semiconductor technology, but physical laws limits the possibilities to reduce the power in the line driver.
In WO99/18662 reduced power dissipation is achieved by using several power supplies to the line driver. In the first embodiment two different positive power supplies are used, which provide power at first and second levels, respectively, where the second level is greater than the first level. A controller causes power to be supplied from the first power supply to the line driver when the magnitude of the input voltage is less than or equal to a predetermined threshold. When the magnitude of the input voltage is greater than the threshold, the controller causes power to be supplied from the second power supply to the line driver.
The problem with this embodiment is that when the amplifier is in an idle mode, it will take an idle voltage in the middle of the voltage range. Idle voltage is in the present description defined as the voltage that is received on the output of the line driver when there is no input signal to it. This is mainly applicable in circuits that are connected differential or in circuits that are AC-connected.
Thus, if the power supply voltage presently used is 5V, then the idle voltage will be 2,5V and if the power supply voltage presently used is 12V, then the idle voltage will be 6V. Hence, the idle voltage differs depending on which power supply voltage it is that is presently used. This is bad, because then the output voltage will change when the power supply voltage is changed, even though it is supposed to be an idle mode. Another problem is that it is necessary to use two different power supplies, which is expensive, inefficient and place consuming.
The second embodiment in WO99/18662 uses four power supplies, two positive and two negative of corresponding values. This makes the idle voltage at zero at all times. The problem with this embodiment is that as many as four different power supplies are needed.
The purpose with the present invention is to provide a line driver, such as a line driver in a multi carrier system, with a low power dissipation and a stable idle voltage without having. to use a lot of different power supplies.
The problems mentioned above with the different embodiments WO99/18662 are solved by defining a voltage range, within which it is the greatest probability that the input voltage to the line driver will fall. A power supply to the line driver is chosen accordingly and whole or part of the power supply voltage is used for generating the output voltage as long as the input voltage is within said range.
Further, a capacitor is included in the line driver and is loaded to a capacitor voltage. Whole or part of said capacitor voltage may then be used in addition to whole or part of the power supply voltage to generate the output voltage when the input voltage is outside said range.
The advantages are that a low power dissipation and a stable idle voltage is achieved in a simple circuit without the need for many power supplies. The larger the differences of probability are within the range compared to outside the range the larger is the gain of lowered power dissipation. This is particularly evident in e.g. systems with Gaussian distributed input voltage probabilities, such as is the case for a line driver in a multicarrier system.